Constraint on affine model motion vector

ABSTRACT

In a method for video coding, prediction information of a current block in a current picture is decoded from a coded video bitstream. The prediction information is indicative of an affine model, and the current block includes two or more control points. A motion vector for each of the two or more control points is determined based on a corresponding motion vector predictor for the respective control point. The corresponding motion vector predictor for the respective control point is a first predictor of a plurality of candidate motion vector predictors in a candidate list and meets a constraint that is associated with a motion vector of the corresponding motion vector predictor. Further, parameters of the affine model are defined based on the determined motion vectors of the two or more control points, and at least a sample of the block is reconstructed according to the affine model.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/767,275, “CONSTRAINT ON AFFINE MOTIONMODEL MOTION VECTOR” filed on Nov. 14, 2018, which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry.

According to an aspect of the current disclosure, a method for videocoding in a decoder is provided. In the disclosed method, predictioninformation of a current block in a current picture is decoded from acoded video bitstream. The prediction information is indicative of anaffine model and the current block includes two or more control points.A motion vector for each of the two or more control points issubsequently determined based on a corresponding motion vector predictorfor the respective control point. The corresponding motion vectorpredictor for the respective control point is a first predictor of aplurality of candidate motion vector predictors in a candidate list, andmeets a constraint that is associated with a motion vector of thecorresponding motion vector predictor. Further, parameters of the affinemodel are determined based on the determined motion vectors of the twoor more control points. The parameters of the affine model are used totransform between the block and a reference block in a reference picturethat has been reconstructed. At least a sample of the block issubsequently reconstructed according to the affine model.

The disclosed method further includes applying the constraint that isreceived in the coded video bitstream. The coded video bitstream is atleast one of a sequence parameter set, a picture parameter set, and aslice header. The method also includes applying the constraint that ispredefined for determining the motion vector for each of the two or morecontrol points.

In some embodiments, a second predictor of the plurality of candidatemotion vector predictors that exceeds the constraint is removed in thecandidate list. In some embodiments, the second predictor that exceedsthe constraint is replaced with a new predictor in the candidate list.In some embodiments, a motion vector of the second predictor is clippedso that the second predictor meets the constraint.

In some embodiments, the constraint indicates a first limit. The firstlimit is applied on a horizontal component of a motion vector differencebetween the motion vector of one of the two or more control points ofthe block and a motion vector prediction of the one of the two or morecontrol points of the block. The motion vector prediction is determinedbased on the corresponding motion vector predictor of the one of the twoor more control points. The constraint can also indicate a second limit.The second limit is applied on a vertical component of the motion vectordifference between the motion vector of the one of the two or morecontrol points of the block and the motion vector prediction of the oneof the two or more control points of the block. The motion vectorprediction is determined based on the corresponding motion vectorpredictor of the one of the two or more control points.

In some embodiments, the constraint indicates a third limit. The thirdlimit is applied on a horizontal component of a motion vector that isassociated with a control point of the corresponding motion vectorpredictor of the one of the two or more control points. The constraintcan also indicate a fourth limit applied on a vertical component of themotion vector that is associated with the control point of thecorresponding motion vector predictor of the one of the two or morecontrol points.

In some embodiments, the constraint indicates a fifth limit. The fifthlimit is associated with a first luma sample positon. The first lumasample position is referred to by a motion vector associated with acontrol point of the corresponding motion vector predictor of the one ofthe two or more control points. The fifth limit is defined by a firstnumber of luma samples beyond the width picture boundary of the currentpicture. The constraint can also indicate a sixth limit. The six limitis associated with a second luma sample position. The second luma sampleposition is referred to by the motion vector associated with the controlpoint of the corresponding motion vector predictor of the one of the twoor more control points. The sixth limit can be defined by a secondnumber of luma samples beyond the height picture boundary of the currentpicture.

In some embodiments, the fifth limit is a first percentage of a heightof the current picture, and the sixth limit is a second percentage of awidth of the current picture. In some embodiments, the fifth limit isdifferent from the sixth limit.

In the disclosed method, a first ratio R1 is equal to(|MV1x−MV0x|/W,|MV1y−MV0y|/W), and a second ratio R2 is equal to(|MV2x−MV0x|/H,|MV2y−MV0y|/H). MV0x is a horizontal component of amotion vector of a first control point of the two or more control pointsfor the block. MV1x is a horizontal component of a motion vector of asecond control point of the two or more control points for the block.MV2x is a horizontal component of a motion vector of a third controlpoint of the two or more control points for the block. MV0x is avertical component of the motion vector of the first control point ofthe two or more control points for the block. MV1y is a verticalcomponent of the motion vector of the second control point of the two ormore control points for the block. MV2y is a vertical component of themotion vector of the third control point of the two or more controlpoints for the block.

Further, |MV1x−MV0x|/W is a horizontal component of the first ratio R1.|MV1y−MV0y|/W is a vertical component of the first ratio R1.|MV2x−MV0x|/H is a horizontal component of the second ratio R2.|MV2y−MV0y|/H is a vertical component of the second ratio R2.Accordingly, the constraint indicates a first threshold applied to amaximum value of the horizontal component and the vertical component ofthe first ratio R1. The constraint can also indicate a second thresholdapplied to a maximum value of the horizontal component and the verticalcomponent of the second ratio R2. The constraint can indicate a thirdthreshold applied to a minimum value of the horizontal component and thevertical component of the first ratio R1. The constraint can furtherindicate a fourth threshold applied to a minimum value of the horizontalcomponent and the vertical component of the second ratio R2.

In some embodiments, the first threshold is different from the secondthreshold, and the third threshold is different from the fourththreshold.

According to another aspect of the present disclosure, an apparatus isprovided. The apparatus has processing circuitry. The processingcircuitry is configured to perform the disclosed method for videocoding.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 shows an example of spatial and temporal candidates.

FIG. 9 shows an example of an affine coded block.

FIG. 10 shows examples of 6-parameter and 4-parameter affine models.

FIG. 11 shows a mode based inherited affine prediction according to anembodiment.

FIG. 12A shows a first example of a control point based constructedaffine prediction.

FIG. 12B shows a second example of a control point based constructedaffine prediction.

FIG. 13 shows a diagram of a 4-parameter affine motion model accordingto an embodiment.

FIG. 14 shows a diagram of a 6-parameter affine motion model accordingto an embodiment.

FIG. 15 shows a first sample based out-of-picture constraint for controlpoint motion vector (CPMV) of affine predictors according to anembodiment.

FIG. 16 shows a second sample based out-of-picture constraint for CPMVof affine predictors according to an embodiment.

FIG. 17 shows a first percentage based out-of-picture constraint forCPMV of affine predictors according to an embodiment.

FIG. 18 shows a second percentage based out-of-picture constraint forCPMV of affine predictors according to an embodiment.

FIG. 19 shows a flow chart outlining a process example according to someembodiments of the disclosure.

FIG. 20 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure are directed to affine merge and affine motionvector coding. The disclosed methods can be used in advanced videocodecs (e.g., AVC) to improve the coding performance of affine interprediction. A motion vector can refer to a block mode, in which onewhole block uses a set of motion information, such as the mergecandidates in the HEVC standard. Further, the motion vector can refer toa sub-block mode, in which different sets of motion information mayapply for different parts of the block, such as in an affine mode and anadvanced temporal MV prediction (ATMVP) in the VVC standard.

Generally, a motion vector for a block can be coded either in anexplicit way, to signal the difference to a motion vector predictor(e.g., advanced motion vector prediction or AMVP mode); or in animplicit way, to be indicated completely from one previously coded orgenerated motion vector. The later one is referred to as a merge mode,meaning the current block is merged into a previously coded block byusing its motion information.

In both the AMVP mode and the merge mode, a candidate list isconstructed during decoding. FIG. 8 shows examples of spatial andtemporal candidates. For the merge mode in the inter prediction, mergecandidates in a merge candidate list can be formed by checking motioninformation from either spatial and/or temporal neighboring blocks ofthe current block. In the FIG. 8 example, spatial candidate blocks A1,B1, B0, A0, and B2 are sequentially checked. When one or more of thespatial candidate blocks are valid candidates (e.g., are coded withmotion vectors), the motion information of the one or more validcandidate blocks can be added into the merge candidate list. A pruningoperation can be performed to ensure that duplicated candidates are notincluded in the merge candidate list, e.g., are not added into the listagain. The candidate blocks A1, B1, B0, A0, and B2 are adjacent tocorners of the current block, and can be referred to as cornercandidates.

After the spatial candidates are checked, temporal candidates can bechecked for inclusion into the list. In some examples, a co-locatedblock of the current block is found in a specified reference picture.The motion information at the C0 position (e.g., at the bottom rightcorner of the current block) of the co-located block can be used as atemporal merge candidate when available. If the block at this positionis not coded in inter mode or is not otherwise available, the C1position (e.g., at the bottom right corner adjacent to the center of theco-located block) can be used instead. The present disclosure providestechniques to further improve the merge mode.

In the advanced motion vector prediction (AMVP) mode, motion informationof spatial and temporal neighboring blocks can be used to predict themotion information of the current block. The prediction residue isfurther coded. Examples of the spatial and temporal neighboringcandidates are illustrated in FIG. 8.

In some embodiments, a two-candidate motion vector predictor list isformed in the AMVP mode. For example, the two-candidate motion vectorpredictor list includes a first candidate predictor and a secondcandidate predictor. The first candidate predictor is an availablemotion vector from the left edge, for example a first available motionvector in the order of spatial A0, A1 positions. The second candidatepredictor is an available motion vector from the top edge, for example afirst available motion vector in the order of spatial B0, B1 and B2positions. If a valid motion vector cannot be found from the checkedlocations (e.g., for the left edge and the top edge), none of thecandidate predictors will be added to the list. If the two candidatepredictors are available and are the same, only one will be kept in thelist. If the list is not full (with two different candidates), thetemporal co-located motion vector (after scaling) from the C0 locationcan be used as another candidate. If motion information at the C0location is not available, the C1 location can be used instead.

In some examples, if there are still not enough motion vector predictorcandidates, a zero motion vector will be used to fill up the list.

According to an aspect of the disclosure, affine motion compensation, bydescribing a 6-parameter (or a simplified 4-parameter) affine model fora coding block, can efficiently predict the motion information forsamples within the current block. More specifically, in an affine codedor described coding block, different parts of the samples can havedifferent motion vectors. The basic unit having a motion vector in anaffine coded or described block can be referred to as a sub-block. Thesize of the sub-block can be as small as 1 sample only; and can be aslarge as the size of current block.

In an affine mode, a motion vector (relative to the targeted referencepicture) can be derived for each sample in the current block using amodel such as the 6 parameter affine motion model or 4 parameter affinemotion model. In order to reduce implementation complexity, affinemotion compensation can be performed on a sub-block basis, instead of ona sample basis. That means, a motion vector will be derived for eachsub-block the motion vector is the same for samples in a respectivesub-block. A specific location of each sub-block can be assumed, such asthe top-left or the center point of the sub-block, to be arepresentative location. In one example, such a sub-block size contains4×4 samples.

An affine motion model can have 6 parameters to describe the motioninformation of a block. After the affine transformation, a rectangularblock will become a parallelogram. In an example, the 6 parameters of anaffine coded block can be represented by 3 motion vectors at threedifferent locations of the block. FIG. 8 shows an example in which 3corners of the block can be used. The locations of the corners in FIG. 8can be referred to as control ponints.

FIG. 9 shows an example of an affine coded block (900). The block (900)is represented by motion vectors {right arrow over (ν₀)}, {right arrowover (ν₁)}, and {right arrow over (ν₀)} at three corner locations A, B,and C to describe the motion information of the affine motion model usedfor the block (900). As described above, these locations A, B, and C canbe referred to as control points.

An affine motion model can use 4 parameters to describe the motioninformation of a block based on an assumption that after the affinetransformation, the shape of the block does not change. Therefore, arectangular block will remain rectangular and with the same aspect ratio(e.g., height/width) after the transformation. The affine motion modelof such a block can be represented by two motion vectors at twodifferent locations, such as at the corner locations A and B.

FIG. 10 shows examples of affine transformations for a 6-parameteraffine mode (using a 6-parameter affine model) and a 4-parameter affinemode (using a 4-parameter affine model). When assumptions are made suchthat the object only has zooming and translational motions, or theobject only has rotation and translation models, then the affine motionmodel can be further simplified to a 3-parameter affine motion modelwith 2 parameters to indicate the translational part and 1 parameter toindicate either a scaling factor for zooming or an angular factor forrotation.

According to an aspect of the disclosure, when affine motioncompensation is used, two signaling techniques can be used. The twosignaling techniques can be referred to as a merge mode based signalingtechnique and a residue (AMVP) mode based signaling technique.

In the merge mode, the affine information of the current block ispredicted from previously affine coded blocks. In one method, thecurrent block is assumed to be in the same affine object as thereference block, so that the MVs at the control points of the currentblock can be derived from a model of the reference block. The MVs atother locations of the current block can be linearly modified in thesame way as from one control point to another in the reference block.This method can be referred to as a model based affine prediction. Anexample of the model based affine prediction, or model based inheritedaffine prediction, is illustrated in FIG. 11.

In another method, motion vectors of neighboring blocks can be useddirectly as the motion vectors at control points of a current block.Motion vectors for the rest of the block can then be generated usinginformation from the control points. This method can be referred to ascontrol point based constructed affine prediction. In either method, noresidue components of the MVs at current block are to be signaled. Inother words, the residue components of the MVs are assumed to be zero.An example of the control point based affine prediction is illustratedin FIGS. 12A and 12B.

For the residue (AMVP) mode based signaling technique, affineparameters, or the MVs at the control points of the current block, areto be predicted. Because there is more than one motion vector to bepredicted, the candidate list for motion vectors at the control points(e.g., all the control points) is organized in grouped way such thateach candidate in the list includes a set of motion vector predictorsfor the control points. For example, candidate 1={predictor for controlpoint A, predictor for control point B, predictor for control point C};candidate 2={predictor for control point A, predictor for control pointB, predictor for control point C}, etc. The predictor for the samecontrol point in different candidates can be the same or different. Themotion vector predictor flag ((mvp_10_flag for List 0 or mvp_11_flag forList 1) can be used to indicate which candidate from the list is chosen.After prediction, the residue part of the parameter, or the differencesof the actual MVs to the MV predictors at the control points, are to besignaled. The MV predictor at each control point can also be derivedfrom model based affine prediction from one of its neighbors, using themethod described from the above description for merge mode basedsignaling technique.

The method can be illustrated based on a 4-parameter affine model with 2control points (e.g., CP0 and CP1), as shown in FIG. 13. However, FIG.13 is a merely example and the methods in the disclosure can be extendedto other motion models, or affine models with different numbers ofparameters. In some embodiments, the model used may not always be anaffine model, but other types of motion.

In an example, a 4-parameter affine model is described, such as shown byequation (1)

$\begin{matrix}\{ {\begin{matrix}{x^{\prime} = {{{\rho cos\theta} \cdot x} + {p\; \sin \; {\theta \cdot y}} + c}} \\{y^{\prime} = {{{- {\rho sin\theta}} \cdot x} + {p\; \cos \; {\theta \cdot y}} + f}}\end{matrix},}  & (1)\end{matrix}$

where ρ is the scaling factor for zooming, θ is the angular factor forrotation, and (c, f) is the motion vector to describe the translationalmotion. (x, y) is a pixel location in the current picture, (x′, y′) is acorresponding pixel location in the reference picture.

Let a=ρ cos θ, and let b=ρ sin θ, equation (1) may become the followingform as in equation (2)

$\begin{matrix}\{ \begin{matrix}{x^{\prime} = {{a \cdot x} + {b \cdot y} + c}} \\{y^{\prime} = {{{- b} \cdot x} + {a \cdot y} + f}}\end{matrix}  & (2)\end{matrix}$

Thus, a 4-parameter affine model can be represented by a set ofmodel-based parameters {ρ,θ,c,f}, or {a,b,c,f }. Based on Eq. 2, motionvector (MV_(x),MV_(y)) at a pixel position (x, y) can be described as inequation (3).

$\begin{matrix}\{ {\begin{matrix}{{MV}_{x} = {{x^{\prime} - x} = {{ax} + {by} + c}}} \\{{MV}_{y} = {{y^{\prime} - y} = {{- {bx}} + {ay} + f}}}\end{matrix},}  & (3)\end{matrix}$

where V_(x) is a horizontal motion vector value, and V_(y) is a verticalmotion vector value.

The 4-parameter affine model can also be represented by the motionvectors of two control points, CP0 and CP1, of the block. Similarly,three control points may be required to represent a 6-parameter affinemodel. To derive the motion vector at position (x, y) in the currentblock, a following equation (4) can be used:

$\begin{matrix}\{ {\begin{matrix}{v_{x} = {{\frac{( {v_{1x} - v_{0x}} )}{w}x} - {\frac{( {v_{1y} - v_{0y}} )}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{( {v_{1y} - v_{0y}} )}{w}x} + {\frac{( {v_{1x} - v_{0x}} )}{w}y} + v_{0y}}}\end{matrix},}  & (4)\end{matrix}$

where (ν_(0x), ν_(0y)) is a motion vector of the top-left corner controlpoint, CP0 as depicted in FIG. 13, and (ν_(1x), ν_(1y)) is a motionvector of the top-right corner control point, CP1 as depicted in FIG.13. (ν_(0x), ν_(0y)) and (ν_(1x), ν_(1y)) can also be referred to ascontrol point motion vectors (CMPWs), such as CPMV₀ (ν_(0x), ν_(0y)) andCPMV₁ (ν_(1x), ν_(1y)). Accordingly, in the control-point based model,the affine model of the block can be represented by{ν_(0x),ν_(0y),ν_(1x),ν_(1y),}, or {CPMV₀, CMPV₁}.

Similarly, three control points can be required to represent a6-parameter affine model, including CP0, CP1, and CP2, as depicted inFIG. 14. And alternatively, the 6-parameter affine model can bedescribed in the following equation (5).

$\begin{matrix}\{ \begin{matrix}{x^{\prime} = {{a \cdot x} + {b \cdot y} + c}} \\{y^{\prime} = {{d \cdot x} + {e \cdot y} + f}}\end{matrix}  & (5)\end{matrix}$

And motion vector values at position (x, y) in the block can bedescribed by equation (6).

$\begin{matrix}\{ \begin{matrix}{{MV}_{x} = {{x^{\prime} - x} = {{( {a - 1} )x} + {by} + c}}} \\{{MV}_{y} = {{y^{\prime} - y} = {{dx} + {( {e - 1} )y} + f}}}\end{matrix}  & (6)\end{matrix}$

The 6-parameter affine model can also be represented by control pointmotion vectors, such as {CPMV₀, CPMV₁, CPMV₂}.

In methods of deriving affine merge/AMVP predictors, the differenceamong control point motion vectors (CPMVs) of a block can be very large,especially when the affine merge/AMVP predictors are derived by usingcontrol point based constructed affine prediction. In such a case, theaffine parameters derived based on the CPMVs can become very large,which can be interpreted as very large affine transformations, such aszooming or warping. When the affine parameters reach a certain range,the corresponding affine transformation can become impractical in videocoding, and the derived CPMV values can be out of the reasonable range,such as pointing to locations far beyond picture boundaries, orrequiring too many bits to encode/decode the motion vector differencewhen the affine AMVP mode is applied. In addition, the large CPMVs, aswell as other MVs in affine coded sub-blocks (which are derived fromCPMVs), can be used as motion vector predictors in the later codingblock. Too large or impractical MV predictors can also cause issues.

In the disclosure, methods are developed to constrain the range of CPMVsin affine motion compensation to avoid generating invalid or impracticalpredictors. For example, some constraints (limits) can be added to therange of a CPMV. In an embodiment, a constraint for motion vectordifference (MVD) coding is set. In one example, both a MVD oftranslational motion and a MVD of affine motion are constrained. Inanother example, only one of the MVD of translational motion or the MVDof affine motion is constrained. The range of MVD can be constrained toa predefined value (for each MVD component), or a certain number ofbits, such as 31 bits. Alternatively, the range of MVD can be signaledin a bitstream, such as in a sequence parameter set (SPS), pictureparameter set (PPS), or slice header. The constraint can be a conformingconstraint such that a bitstream which contains a MVD beyond theconstraint is regarded as an invalid bitstream. Alternatively, when theMVD is beyond the range, the MVD can be clipped by the rangeaccordingly.

A threshold can set to limit value ranges of motion vector predictorsfor the CPMVs in affine motion compensation. The threshold can set tolimit affine parameter values of affine predictors.

In the disclosure, the proposed methods can be used separately orcombined in any order. The term “block” in the disclosure can beinterpreted as a prediction block, a coding block, or a coding unit(i.e., CU).

When the affine AMVP mode is applied, a constraint can be applied onmotion vector difference coding for affine CPMVs and for translationalMVs. In addition, a constraint can be applied on a motion vectordifference (MVD) for each control point by using a predefined limit.

The motion vector difference can be obtained based on the differencebetween an optimal CPMV of each control point and the correspondingCPMV's predictor (CPMVP). For example, for control point 0, the motionvector difference may be calculated as in equation (7):

MVD₀=CPMV₀−CPMVP₀   (7)

Each CPMV for a respective control point in the block, which is derivedbased on the corresponding affine predictor, can have a horizontalcomponent and a vertical component. Accordingly, a horizontal componentof the motion vector difference for control point 0 can be calculated asin equation (8):

MVDx₀=CPMVx₀−CPMVPx₀   (8)

A vertical component of the motion vector difference for control point 0can be calculated as in equation (9):

MVDy₀=CPMVy₀−CPMVPy₀   (9)

MVDs for control point 1 and/or control point 2 can be calculated in asimilar procedure that is described in equations (7)-(9).

In one embodiment, the number of bits used to represent the horizontalcomponent or vertical component of a control point's MVD can be limitedto be a predefined range. In one example, the limit can be set to be Nbits, such as N=15. Accordingly, for a affine motion predictor of ablock, if the number of bits required to represent a horizontal or avertical component of a MVD of any control point exceeds the predefinedlimit (e.g., 15 bits), the predictor can be pruned or cannot be used asan affine AMVP predictor. In another example, for any control point ofthe block that is under a certain MVD coding precision (e.g., ¼ sampleprecision), when abs(MVDx) of the control point is >=2¹⁵ or abs(MVDy) ofthe control point is >=2¹⁵, the corresponding affine predictor cannot beused as an affine CPMV predictor in the affine AMVP mode. abs(x) hereinmeans an absolute value of x.

It should be noted that the predefined limit mentioned above can be anyvalue, which is not limited by the aforementioned example. Further, theproposed methods mentioned above for MVD constraints can also be appliedto translational (regular) MVs.

When the affine merge mode or the affine AMVP mode is applied, aconstraint can be applied on the value ranges of the affine CMPVpredictors (affine predictor), where values of the derived affine CPMVpredictors can be constrained by a predefined limit.

In one embodiment, the value of the horizontal component or the verticalcomponent of any CPMV of the affine predictor can be constrained by apredefined limit. In one example, the limit can be set to be N bits,such as N=16. When a certain motion vector storage precision is applied(e.g., 1/16 sample precision), if the horizontal component or thevertical component of any CPMV of the affine CPMV predictor is largerthan or equal to 2¹⁶ in an absolute value, the predictor can be prunedor cannot be used as an affine CPMV predictor in the affine AMVP mode.The predefined limit of motion vector can be any value, which is notlimited by the example mentioned above.

In one embodiment, a luma sample position that any CPMV of the affinepredictor is pointing to can be limited to a predefined range. The limitcan be set to a predefined number of luma samples beyond each edge ofthe current picture's boundary. In one example, as shown in FIG. 15, thelimit can be set to be N luma samples, and N can be 128. If any CPMV ofthe affine CPMV predictor points to more than 128 luma samples beyondany edge of the current picture, the corresponding affine predictor canbe pruned and cannot be added to the predictor list. For example, asshown in FIG. 15, all CPMVs of affine predictor A point to positionswithin the limited range, thus affine predictor A can be a validpredictor. However, one CPMV of the affine predictor B points to aposition beyond the limited range (e.g., 128 luma samples), thus theaffine predictor B is invalid, and cannot be added to the predictorlist.

In one embodiment, the luma sample position that any CPMV of the affinepredictor is pointing to can be limited to different predefined ranges(limits). The ranges can be different on a vertical direction and ahorizontal direction. An example can be illustrated in FIG. 16, wherethe limit in the horizontal direction has a predefined value of 256 lumasamples that is different from the limit in the vertical direction whichis 128 luma samples. It should be noted that FIG. 16 is merely anexample and the number of luma samples used as the constraint on thevertical and/or horizontal directions can be any predefined values.

The luma sample position that any CPMV of the affine predictor ispointing to can also be limited to a predefined percentage of widthand/or height of the current picture outside of the corresponding edgesof the current picture boundary. In one example, a same percentage canbe applied on both the current picture width along the horizontaldirection and current picture height along the vertical direction. Asshown in FIG. 17, the limit can be set to be 25% of the picture width inthe horizontal direction, and 25% of the picture height in the verticaldirection. FIG. 17 is a merely example, the number of percentage used asthe constraint on the vertical and/or horizontal directions can be anypredefined values.

Further, different percentages can be applied on the current picturewidth along the horizontal direction and on the current picture heightalong the vertical direction. As shown in FIG. 18, the limit can be setto 25% of the picture width in the horizontal direction, and 20% of thepicture height in the vertical direction. The percentages used as theconstraint in the vertical and/or horizontal directions in FIG. 18 aremerely examples, and the percentages can be any predefined values.

When constraints are applied on affine parameter values of the affinepredictors, affine predictors violating the constraints can beeliminated from the predictor list. Exemplary constraints can bedescribed based on the affine merge mode. According to the affine modelequations (3) and (6), the CPMV values of CP0 represents thetranslational MV of the block, and CPMVs of CP1/CP2 reflects the shapetransformation from affine model plus the translational model. Let W andH denote the current block's width and height, respectively.

In the 6-parameter case, the control point motion vectors can becalculated as:

CPMV0: (c,f),

CPMV1: ((a−1)*W+c,d*W+f), and

CPMV2: (b*H+c,(e−1)*H+f).

In the 4-parameter case, the control point motion vectors can becalculated as:

CPMV0: (c,f), and

CPMV1: ((a−1)*W+c,−b*w+f).

The delta (difference) between CP1 and CP0, CP2 and CP0, can representthe affine transformation part of the affine model, and the delta valuescan fall in a reasonable range. Let D1 denote the delta between CPMV1and CPMV0, which is:

D1=CPMV1−CPMV0=((a−1)*W+c,d*W+f)−(c,f)=((a−1)*W,d*W)   (10)

In the 6-parameter model case, let D2 denote the delta between CPMV2 andCPMV0, which is:

D2=CPMV2−CPMV0=(b*H+c,(e−1)*H+f)−(c,f)=(b*H,(e−1)*H)   (11)

Since D1 only includes affine parameter a, d, and block width W, a ratioR1 can be defined to represent the affine parameter value range in D1,where

R1=|D1/W|=(|a−1|,|d|)   (12)

Similarly, since D2 only includes affine parameter b, e, and blockheight H, a ratio R2 can be defined to represent the affine parametervalue range in D2, where

R2=|D2/W|=(|b|,|e−1|)   (13)

R1 and R2 can also be represented by CPMV values, as in followingequations:

R1=abs(CPMV1−CPMV0)/W=(|MV1x−MV0x|/W,|MV1y−MV0y|/W)   (14)

R2=abs(CPMV2−CPMV0)/H=(|MV2x−MV0x|/H,|MV2y−MV0y|/H)   (15)

In the proposed method, constraints can be applied to R1 and/or R2 asthe constraint for the affine CPMV predictors. In the 4-parameter affinemodel, a predefined threshold can be applied on R1. In the 6-parameteraffine model, a predefined threshold can be applied on R1 and/or R2. Theratio R1 and/or R2 can be denoted in a generalized form R. The disclosedmethod can be used for the 4-parameter affine model and the 6-parameteraffine model, whenever the method is applicable. When the constraint isviolated, the corresponding affine predictor can be excluded from thepredictor list.

In an embodiment, a predefined threshold can be applied on a maximumvalue from the horizontal and the vertical components of ratio R. In anexample, the threshold value can be set to be 256 with the MV storageprecision, such as 1/16 pixel accuracy. Accordingly, the threshold valueis equal to 16 pixel. For a 6-parameter affine predictor with CPMV0(MV0x, MV0y), CPMV1(MV1x, MV1y), and CPMV2(MV2x, MV2y), the ratio R1 andR2 can be described in equations (16) and (17):

R1=(|MV1x−MV 0 x|/W,|MV1y−MV0y|/W), and   (16)

R2=(|MV2x−MV0x|/H,|MV2y−MV0y|/H)   (17)

For R1, the maximum component, which is denoted asmax(|MV1x−MV0x|/W,|MV1y−MV0y|/W), can be checked against the thresholdvalue. For R2, the maximum component, which is denoted asmax(|MV2x−MV0x|/H,|MV2y−MV0y|/H), can also be check against thethreshold value. When any of the following conditions (a) and (b) istrue, the affine predictor can be considered invalid and cannot be addedto the final predictor list.

max(|MV1x−MV0x|/W,|MV1y−MV0y|/W)>256   (a)

max(|MV2x−MV0x|/H,|MV2y−MV0y|/H)>256   (b)

It should be noted that the predefined threshold value is not limited tothe above example. For example, the predefined threshold can be definedas a value larger than 8. Under the 1/16 pixel accuracy, the predefinedthreshold accordingly is equal to ½ pixel.

In some embodiments, a predefined limit can be applied on a minimumvalue from the horizontal and the vertical components of ratio R.Different predefined limits can be applied on the horizontal componentof ratio R and the vertical component of ratio R. Further, the limitsmay be signaled in bitstreams, such as in SPS, PPS, or slice header.

FIG. 19 shows a flow chart outlining a process (1900) according to anembodiment of the disclosure. The process (1900) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1900) are executed by processing circuitry,such as the processing circuitry in the terminal devices (210), (220),(230) and (240), the processing circuitry that performs functions of thevideo encoder (303), the processing circuitry that performs functions ofthe video decoder (310), the processing circuitry that performsfunctions of the video decoder (410), the processing circuitry thatperforms functions of the video encoder (503), and the like. In someembodiments, the process (1900) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1900). The process startsat (S1901) and proceeds to (S1910).

At (S1910), prediction information of a block in a current picture canbe decoded from a coded video bitstream. The prediction informationincludes a plurality of offset indices for prediction offsets associatedwith an affine model in an inter prediction mode. The block includes twoor more control points.

At (S1920), a motion vector for each of the two or more control pointscan be determined based on a corresponding motion vector predictor forthe respective control point. The corresponding motion vector predictorfor the respective control point can be a first predictor of a pluralityof candidate motion vector predictors in a candidate list and meets asignaled constraint that is associated with a motion vector of thecorresponding motion vector predictor. The signaled constraint can bereceived with the coded video bitstream. For example, the constraint maybe signaled in the SPS, PPS, or slice header.

At (S1930), parameters of the affine model can be determined based onthe determined motion vectors of the two or more control points. Theparameters of the affine model can be used to transform between theblock and a reference block in a reference picture that has beenreconstructed.

At (S1940), samples of the block are reconstructed according to theaffine model. In an example, a reference pixel in the reference picturethat corresponds to a pixel in the block is determined according to theaffine model. Further, the pixel in the block is reconstructed accordingto the reference pixel in the reference picture. Then, the processproceeds to (S1999) and terminates.

In the disclosure, the proposed methods can be used separately orcombined in any order. Further, the methods (or embodiments) may beimplemented by processing circuitry (e.g., one or more processors or oneor more integrated circuits). In an example, the one or more processorsexecute a program that is stored in a non-transitory computer-readablemedium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 20 shows a computersystem (2000) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 20 for computer system (2000) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (2000).

Computer system (2000) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (2001), mouse (2002), trackpad (2003), touchscreen (2010), data-glove (not shown), joystick (2005), microphone(2006), scanner (2007), camera (2008).

Computer system (2000) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (2010), data-glove (not shown), or joystick (2005), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (2009), headphones(not depicted)), visual output devices (such as screens (2010) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (2000) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(2020) with CD/DVD or the like media (2021), thumb-drive (2022),removable hard drive or solid state drive (2023), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (2000) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (2049) (such as, for example USB ports of thecomputer system (2000)); others are commonly integrated into the core ofthe computer system (2000) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (2000) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (2040) of thecomputer system (2000).

The core (2040) can include one or more Central Processing Units (CPU)(2041), Graphics Processing Units (GPU) (2042), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(2043), hardware accelerators for certain tasks (2044), and so forth.These devices, along with Read-only memory (ROM) (2045), Random-accessmemory (2046), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (2047), may be connectedthrough a system bus (2048). In some computer systems, the system bus(2048) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (2048),or through a peripheral bus (2049). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (2041), GPUs (2042), FPGAs (2043), and accelerators (2044) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(2045) or RAM (2046). Transitional data can be also be stored in RAM(2046), whereas permanent data can be stored for example, in theinternal mass storage (2047). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (2041), GPU (2042), massstorage (2047), ROM (2045), RAM (2046), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (2000), and specifically the core (2040) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (2040) that are of non-transitorynature, such as core-internal mass storage (2047) or ROM (2045). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (2040). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(2040) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (2046) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (2044)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder,comprising: decoding prediction information of a current block in acurrent picture from a coded video bitstream, the prediction informationbeing indicative of an affine model, the current block including two ormore control points; determining a motion vector for each of the two ormore control points based on a corresponding motion vector predictor forthe respective control point, the corresponding motion vector predictorfor the respective control point being a first predictor of a pluralityof candidate motion vector predictors in a candidate list and meeting aconstraint that is associated with a motion vector of the correspondingmotion vector predictor; determining parameters of the affine modelbased on the determined motion vectors of the two or more controlpoints, the parameters of the affine model being used to transformbetween the block and a reference block in a reference picture that hasbeen reconstructed; and reconstructing at least a sample of the blockaccording to the affine model.
 2. The method of claim 1, furthercomprising at least one of: applying the constraint that is received inthe coded video bitstream, the coded video bitstream being at least oneof a sequence parameter set, a picture parameter set, and a sliceheader; or applying the constraint that is predefined for determiningthe motion vector for each of the two or more control points.
 3. Themethod of claim 1, further comprising one of: removing a secondpredictor of the plurality of candidate motion vector predictors in thecandidate list that exceeds the constraint; replacing the secondpredictor that exceeds the constraint with a new predictor in thecandidate list; or clipping a motion vector of the second predictor sothat the second predictor meets the constraint.
 4. The method of claim1, wherein the constraint indicates: a first limit applied on ahorizontal component of a motion vector difference between the motionvector of one of the two or more control points of the block and amotion vector prediction of the one of the two or more control points ofthe block, the motion vector prediction being determined based on thecorresponding motion vector predictor of the one of the two or morecontrol points; and a second limit applied on a vertical component ofthe motion vector difference between the motion vector of the one of thetwo or more control points of the block and the motion vector predictionof the one of the two or more control points of the block, the motionvector prediction being determined based on the corresponding motionvector predictor of the one of the two or more control points.
 5. Themethod of claim 1, wherein the constraint indicates: a third limitapplied on a horizontal component of a motion vector that is associatedwith a control point of the corresponding motion vector predictor of theone of the two or more control points; and a fourth limit applied on avertical component of the motion vector that is associated with thecontrol point of the corresponding motion vector predictor of the one ofthe two or more control points.
 6. The method of claim 1, wherein theconstraint indicates: a fifth limit from a width picture boundary for afirst luma sample position to which a motion vector associated with acontrol point of the corresponding motion vector predictor of the one ofthe two or more control points refers, the fifth limit being defined bya first number of luma samples beyond the width picture boundary of thecurrent picture; and a sixth limit from a height picture boundary for asecond luma sample position to which the motion vector associated withthe control point of the corresponding motion vector predictor of theone of the two or more control points refers, the sixth limit beingdefined by a second number of luma samples beyond the height pictureboundary of the current picture.
 7. The method of claim 6, wherein thefifth limit is a first percentage of a height of the current picture,and the sixth limit is a second percentage of a width of the currentpicture.
 8. The method of claim 6, wherein the fifth limit is differentfrom the sixth limit.
 9. The method of claim 1, wherein a first ratioR1=(|MV1x−MV0x|/W,|MV1y−MV0y|/W), and a second ratioR2=(|MV2x−MV0x|/H,|MV2y−MV0y|/H), where: MV0x is a horizontal componentof a motion vector of a first control point of the two or more controlpoints for the block, MV1x is a horizontal component of a motion vectorof a second control point of the two or more control points for theblock, MV2x is a horizontal component of a motion vector of a thirdcontrol point of the two or more control points for the block, MV0x is avertical component of the motion vector of the first control point ofthe two or more control points for the block, MV1y is a verticalcomponent of the motion vector of the second control point of the two ormore control points for the block, MV2y is a vertical component of themotion vector of the third control point of the two or more controlpoints for the block, |MV1x−MV0x|/W is a horizontal component of thefirst ratio R1, |MV1y−MV0y|/W is a vertical component of the first ratioR1, |MV2x−MV0x|/H is a horizontal component of the second ratio R2,|MV2y−MV0y|/H is a vertical component of the second ratio R2, and theconstraint indicates one of: a first threshold applied to a maximumvalue of the horizontal component and the vertical component of thefirst ratio R1; a second threshold applied to a maximum value of thehorizontal component and the vertical component of the second ratio R2;a third threshold applied to a minimum value of the horizontal componentand the vertical component of the first ratio R1; and a fourth thresholdapplied to a minimum value of the horizontal component and the verticalcomponent of the second ratio R2.
 10. The method of claim 9, wherein thefirst threshold is different from the second threshold, and the thirdthreshold is different from the fourth threshold.
 11. An apparatus forvideo decoding, comprising: processing circuitry configured to: decodeprediction information of a current block in a current picture from acoded video bitstream, the prediction information being indicative of anaffine model, the current block including two or more control points;determine a motion vector for each of the two or more control pointsbased on a corresponding motion vector predictor for the respectivecontrol point, the corresponding motion vector predictor for therespective control point being a first predictor of a plurality ofcandidate motion vector predictors in a candidate list and meeting aconstraint that is associated with a motion vector of the correspondingmotion vector predictor; determine parameters of the affine model basedon the determined motion vectors of the two or more control points, theparameters of the affine model being used to transform between the blockand a reference block in a reference picture that has beenreconstructed; and reconstruct at least a sample of the block accordingto the affine model.
 12. The apparatus of claim 11, wherein theprocessing circuitry is configured to perform one of: applying theconstraint that is received in the coded video bitstream, the codedvideo bitstream being at least one of a sequence parameter set, apicture parameter set, and a slice header; or applying the constraintthat is predefined for determining the motion vector for each of the twoor more control points.
 13. The apparatus of claim 11, wherein theprocessing circuitry is configured to operate one of: remove a secondpredictor of the plurality of candidate motion vector predictors in thecandidate list that exceeds the constraint; replace the second predictorthat exceeds the constraint with a new predictor in the candidate list;or clip a motion vector of the second predictor so that the secondpredictor meets the constraint.
 14. The apparatus of claim 11, whereinthe constraint indicates: a first limit applied on a horizontalcomponent of a motion vector difference between the motion vector of oneof the two or more control points of the block and a motion vectorprediction of the one of the two or more control points of the block,the motion vector prediction being determined based on the correspondingmotion vector predictor of the one of the two or more control points;and a second limit applied on a vertical component of the motion vectordifference between the motion vector of the one of the two or morecontrol points of the block and the motion vector prediction of the oneof the two or more control points of the block, the motion vectorprediction being determined based on the corresponding motion vectorpredictor of the one of the two or more control points.
 15. Theapparatus of claim 11, wherein the constraint indicates: a third limitapplied on a horizontal component of a motion vector that is associatedwith a control point of the corresponding motion vector predictor of theone of the two or more control points; and a fourth limit applied on avertical component of the motion vector that is associated with thecontrol point of the corresponding motion vector predictor of the one ofthe two or more control points.
 16. The apparatus of claim 11, whereinthe constraint indicates: a fifth limit from a width picture boundaryfor a first luma sample position to which a motion vector associatedwith a control point of the corresponding motion vector predictor of theone of the two or more control points refers, the fifth limit beingdefined by a first number of luma samples beyond the width pictureboundary of the current picture; and a sixth limit from a height pictureboundary for a second luma sample position to which the motion vectorassociated with the control point of the corresponding motion vectorpredictor of the one of the two or more control points refers, the sixthlimit being defined by a second number of luma samples beyond the heightpicture boundary of the current picture.
 17. The apparatus of claim 16,wherein the fifth limit is a first percentage of a height of the currentpicture, and the sixth limit is a second percentage of a width of thecurrent picture.
 18. The apparatus of claim 11, wherein a first ratioR1=(|MV1x−MV0x|/W,|MV1y−MV0y|/W), and a second ratioR2=(|MV2x−MV0x|/H,|MV2y−MV0y|/H), where: MV0x is a horizontal componentof a motion vector of a first control point of the two or more controlpoints for the block, MV1x is a horizontal component of a motion vectorof a second control point of the two or more control points for theblock, MV2x is a horizontal component of a motion vector of a thirdcontrol point of the two or more control points for the block, MV0x is avertical component of the motion vector of the first control point ofthe two or more control points for the block, MV1y is a verticalcomponent of the motion vector of the second control point of the two ormore control points for the block, MV2y is a vertical component of themotion vector of the third control point of the two or more controlpoints for the block, |MV1x−MV0x|/W is a horizontal component of thefirst ratio R1, |MV1y−MV0y|/W is a vertical component of the first ratioR1, |MV2x−MV0x|/H is a horizontal component of the second ratio R2,|MV2y−MV0y|/H is a vertical component of the second ratio R2, and theconstraint indicates one of: a first threshold applied to a maximumvalue of the horizontal component and the vertical component of thefirst ratio R1; a second threshold applied to a maximum value of thehorizontal component and the vertical component of the second ratio R2;a third threshold applied to a minimum value of the horizontal componentand the vertical component of the first ratio R1; and a fourth thresholdapplied to a minimum value of the horizontal component and the verticalcomponent of the second ratio R2.
 19. The apparatus of claim 18, whereinthe first threshold is different from the second threshold, and thethird threshold is different from the fourth threshold.
 20. Anon-transitory computer-readable medium storing instructions which whenexecuted by a computer for video decoding cause the computer to perform:decoding prediction information of a current block in a current picturefrom a coded video bitstream, the prediction information beingindicative of an affine model, the current block including two or morecontrol points; determining a motion vector for each of the two or morecontrol points based on a corresponding motion vector predictor for therespective control point, the corresponding motion vector predictor forthe respective control point being a first predictor of a plurality ofcandidate motion vector predictors in a candidate list and meeting aconstraint that is associated with a motion vector of the correspondingmotion vector predictor, the constraint being either signaled orpredefined; determining parameters of the affine model based on thedetermined motion vectors of the two or more control points, theparameters of the affine model being used to transform between the blockand a reference block in a reference picture that has beenreconstructed; and reconstructing at least a sample of the blockaccording to the affine model.